1. Field of the Invention
The present invention relates to applications utilizing mono-energetic gamma-rays (MEGa-rays), and more specifically, it relates to techniques that utilize MEGa-rays for characterizing isotopes.
2. Description of Related Art
MEGa-ray sources are created by the scattering of energetic (Joule-class), short-duration (few picosecond) laser pulses off of relativistic electron beams (several hundred MeV). The resulting scattered photons are forwardly directed in a narrow beam (typically milli-radians in divergence), are mono-energetic, tunable, polarized and have peak photon brilliance (photons/second/unit solid angle, per unit area, per unit bandwidth) that exceeds that of the best synchrotrons by over 15 orders of magnitude at gamma-ray energies in excess of 1 MeV. Such beams can efficiently excite the protons in the nucleus of a specific isotope, so called nuclear resonance fluorescence (NRF). NRF resonant energies are a function of the number of protons and neutrons in the nucleus and are thus a unique signature of each isotope. It has been suggested that NRF can be used to identify specific isotopes. It is has been further suggested (T-REX/FINDER) that MEGa-ray sources are ideal for this application and not only enable identification of isotopes but can also be used to determine the quantity and spatial distribution of isotopes in a given object. In order to accomplish these tasks one analyzes the MEGa-ray beam transmitted through a particular object. NRF resonances are narrow, typically 10E-6 wide compared to the resonant energy, e.g., 1 eV wide for a 1 MeV resonant energy. MEGa-ray sources on the other hand are typically 10E-3 wide relative to their carrier energy, e.g., 1 keV wide for a carrier energy of 1 MeV. A given amount of (e.g., grams) of a resonant isotope removes a corresponding amount of resonant photons from a MEGa-ray beam, according to Beer's law. Detection or measurement of the absence of resonant photons in a MEGa-ray beam transmitted through an object can be thus be used to determine not only the presence of the material but also its location and its quantity. To do so requires a detector capable of resolving the number of resonant photons removed by the desired object from the MEGa-ray beans. Known gamma-ray spectroscopy technologies are not capable of resolutions better than 10E-3 in the MeV spectral region and are thus not able to accomplish the task. One method suggested by Bertozzi et al. (Bertozzi patent) envisions using a piece of the material under observation after the object in question to evaluate the removal of NRF resonant photons from the beam.
Let us consider in some detail the Bertozzi suggestion using a specific example, namely the location of U235 hidden within a large container such as that used for a trans-oceanic commerce. The Bertozzi suggestion applies specifically to interrogation with a polychromatic gamma-ray beam such as that produced by a Bremsstrahlung source. Referring to FIG. 1A, in his suggestion, the beam 10 transmitted through the cargo container 12 impinges upon two “detectors”. Transmission detector 14 is an energy collector that measures the total, gamma-rays passing through the object and the first detector 16, which consists of a piece (typically a foil) of the material/isotope that is being sought in the container, i.e., a foil 18 of U235 in this example. The foil of U235 in detector 16 is surrounded by a large area, gamma-ray spectrometer 20 that measures the spectrum of the photons scattered by the U235 foil 18. If U235 is present in the cargo container in quantities greater than a few grams, then the resonant photons will be removed from the interrogating gamma-ray beam and the gamma-ray spectrometer surrounding the foil of U235 will not see any resonant photons. As depicted in FIG. 1B, light scattered by the interrogating foil will consist of non-NRF photons and particles 22 such as Compton scattered photons, Delbruck photons and miscellaneous energetic particles. When beam 10 does not propagate onto any U235 within the container, then in addition to the non-NRF photons and particles, spectroscopy of the scattered light will reveal NRF photons 24. FIG. 2A shows a cargo container 30, that includes U235 material 32 that is interrogated by a polychromatic beam 34 that includes light resonant at the U235 line. As shown in FIG. 2B, spectroscopy of the scattered light shows only non-NRF photons and particles 36 and thereby reveals the absence of NRF photons and thus the presence of U235 material in the container. While this method in principle works, it has some significant limitations, in particular, it requires gamma-ray spectroscopy of the scattered photons to be effective. Gamma-ray spectroscopy is difficult and is accomplished in nearly all cases by collecting one gamma-ray at a time and analyzing the total energy of that photon. This can work for beams that have photons distributed evenly in time, e.g., those coming from a Bremsstrahlung source. However Bremsstrahlung sources have been shown to be ill-suited for transmission based NRF detection schemes due to their wide bandwidth and beam divergence which are both ill-matched to NRF detection requirements (Fruit et al. paper). MEGa-ray beams are well suited to transmission detection due to their narrow bandwidth and low divergence (100× smaller than Bremsstrahlung); however, these sources by their nature produce large bursts of photons, up to 10E10 per pulse at rates of 10's to 100's of times per second. MEGa-ray sources are ill-matched to single photon counting based gamma-ray spectroscopy.
Alternative methods that eliminate the limitations of the Bertozzi method are desirable.